Regarding gasoline engines, harmful components in the exhaust gases have been reduced securely by the strict regulations on the exhaust gases and the technological developments capable of coping with the strict regulations. However, regarding diesel engines, the regulations and the technological developments have been advanced less compared to those of gasoline engines due to the unique circumstances that the harmful components are emitted as particulates (i.e., particulate materials, such as carbonaceous fine particles, sulfuric fine particles like sulfates, and high-molecular weight hydrocarbon fine particles, hereinafter collectively referred to as “PMs.”)
As exhaust-gas purifying apparatuses having been developed so far for diesel engines, the following have been known. For example, the exhaust-gas purifying apparatuses can be roughly divided into trapping (or wall-flow) exhaust-gas purifying apparatuses and open (or straight-flow) exhaust-gas purifying apparatuses. Among these, clogged honeycomb structures made from ceramic (i.e., diesel PMs filters, hereinafter referred to as “DPFs”) have been known as one of the trapping exhaust-gas purifying apparatuses. In the DPFs, the honeycomb structures are clogged at the opposite openings of cells in a checkered manner alternately, for instance. The DPFs comprise inlet cells clogged on the downstream side of exhaust gases, outlet cells neighboring the inlet cells and clogged on the upstream side of the exhaust gases, and filter cellular walls demarcating the inlet cells and the outlet cells. In the DPFs, the exhaust gases are filtered by the pores of the filter cellular walls to collect PMs.
In the DPFs, however, the pressure loss increases as PMs deposit thereon. Accordingly, it is needed to regularly remove deposited PMs to recover the DPFs by certain means. Hence, when the pressure loss increases, deposited PMs have been burned with burners or electric heaters conventionally, thereby recovering the DPFs. However, in this case, the greater the deposition of PMs is, the higher the temperature increases in burning deposited PMs. Consequently, there might arise cases that the DPFs are damaged by thermal stress resulting from such burning.
Hence, continuously regenerative DPFs (an exhaust-gas purifying filter catalyst) have been developed recently. In the continuously regenerative DPFs, a coating layer comprising alumina is formed on the surface of the cellular walls of the DPFs, and a catalytic ingredient such as platinum (Pt) is loaded on the coating layer. In accordance with the continuously regenerative DPFs, since the collected PMs are oxidized and burned by the catalytic reaction of the catalytic ingredient, it is possible to regenerate the DPFs by burning PMs simultaneously with or successively after collecting PMs. Moreover, since the catalytic reaction occurs at relatively low temperatures, and since PMs can be burned when they are collected less, the continuously regenerative DPFs produce an advantage that the thermal stress acting onto the DPFs is so less that the DPFs are inhibited from being damaged.
As this type of exhaust-gas purifying filter catalyst, for example, Japanese Unexamined Patent Publication (KOKAI) No. 09-173866 discloses an exhaust-gas purifying filter catalyst which comprises on the surface of the cellular walls, forming a cellular coating layer comprising activated alumina whose particle diameter is larger than the average pore diameter, on the inside of the pores, coating activated alumina whose particle diameter is smaller than the average pore diameter of the cellular walls and further loading catalytic metal. This exhaust-gas purifying filter catalyst enables to increase the specific surface area of the coating layer as well as to reduce the pressure loss.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 09-220423 discloses an exhaust-gas purifying filter catalyst whose cellular wall exhibits a porosity of from 40 to 65% and an average pore diameter of from 5 to 35 μm, and whose coating layer is formed of a porous oxide. In the porous oxide, particles whose particle diameter is less than the average pore diameter of the cellular wall occupy 90% by weight or more. When such a porous oxide with a large specific surface area is coated, it is possible to form the coating layer not only on the surface of the cellular walls but also on the inner surface of the pores. Moreover, when the coating layer is coated in a fixed amount, it is possible to make the thickness of the coating layer thinner. Accordingly, it is possible to inhibit the pressure loss from increasing.
Further, Japanese Unexamined Patent Publication (KOKAI) No. 6-159037 discloses an exhaust-gas purifying filter catalyst whose coating layer is further loaded with a NOx-sorbing member. With the arrangement, NOx can be sorbed in the NOx-sorbing member. Consequently, when a reducing agent such as light oil is sprayed into the exhaust gas, it is possible to reduce the sorbed NOx to purify.
When a large amount of PMs are emitted densely at a short time from a diesel engine, etc., PMs are deposited on the surface of the cellular walls or on the inside of the pores, and the pressure loss is increased, since it exceeds the purification capacity of the exhaust-gas purifying filter catalyst. Contrary, when the porosity of the cellular walls is intensified in order to suppress increase of the pressure loss, PMs are let through cellular walls and PM collecting efficiency is lowered.
For instance, with regard to a substrate having the same porosity of 60%, when a substrate has more pores with smaller diameter, the pressure loss is increased, while when a substrate has fewer pores with larger diameter, the collecting efficiency is low.
Moreover, it is conventionally used to avoid this problem to control the distribution of pore diameter, however, since the distribution of pore diameter is measured by a press-fit measuring method such as a mercury porosimeter, the actual distribution of pores is not reflected. Namely, in a press-fit measuring method, the measured value is prescribed by the diameter of the narrowest part of the pores, and thus, it differs from the actual pore diameter, and the accuracy is low. Consequently, even the distribution of pore diameter is designed appropriately by using a press-fit measuring method, it is difficult to avoid the above mentioned problem completely.